This invention relates to a network architecture that provides improved aggregation of multiple network devices and a lossless arbitration scheme for Carrier Sense Multiple Access/Collision Detect (CSMA/CD) based Local Area Network (LAN) protocols that has increased bandwidth along with predictable network access.
A device wanting to transmit on a CSMA/CD LAN, such as Institute of Electrical and Electronic Engineers (IEEE) 802.3, first listens to the medium. If the medium is idle, the device begins transmitting immediately. If the medium is not idle, the device waits until the medium becomes idle and remains idle for a defined period of time before beginning transmission. If two or more devices begin transmitting at about the same time, their transmissions collide. The collision is detected and all colliding devices terminate their transmission and wait a random period of time (back off) before attempting to transmit again. Devices attempt repeatedly to transmit each packet until they are either successful or they collide a specified number of times in which case they discard the packet.
The range of times from which the back off time is randomly selected increases after each collision. For example, after a first collision, a device waits either zero or one slot times (an interval of time defined in the 802.3 protocol). After a second collision, the device waits anywhere between zero to three slot times, and so on, up to 1023 slot times.
The protocol has several disadvantages. Bandwidth is wasted each time transmissions collide. Transmission is unreliable as packets that experience an excessive number of collisions ale discarded. Access to the medium can also be quite unfair over short periods of time. Short term access unfairness is the result of the back off algorithm. When the demand for use of the LAN is high and two or more devices collide, the first of the colliding devices to successfully transmit on the medium has an advantage over the other devices in the collision group as the range of times from which the back off time is randomly selected is reset to the minimum each time a device successfully transmits on the medium.
The physical extent of a CSMA/CD LAN is limited by the minimum transmission duration to send a packet. Collisions result in shorter than normal duration transmissions which are called transmission fragments. Transmission fragments are recognized and rejected based on their short duration. The physical extent of a CSMA/CD LAN is limited by the requirement that the propagation delay between any two devices attached to the LAN must be somewhat less than half of the minimum transmission time for a packet.
Connecting devices whose electrical separation is greater than that allowed by a CSMA/CD LAN requires switching and buffering. Such devices are connected to different LANs and the LANs are interconnected with switches which provide the necessary buffering. The switch and packet buffering is complex and expensive to implement.
Accordingly, a need remains for a network architecture and arbitration scheme for CSMA/CD LAN protocols that does not waste bandwidth, is more reliable, provides predictable access and allows devices with greater physical separation to be connected to the same LAN.